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Date: Fri, 21 Jun 2024 11:53:45 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: koachan@...tonmail.com
Cc: "David S. Miller" <davem@...emloft.net>,
Andreas Larsson <andreas@...sler.com>,
Nick Desaulniers <ndesaulniers@...gle.com>,
Bill Wendling <morbo@...gle.com>,
Justin Stitt <justinstitt@...gle.com>, glaubitz@...sik.fu-berlin.de,
sparclinux@...r.kernel.org, linux-kernel@...r.kernel.org,
llvm@...ts.linux.dev
Subject: Re: [PATCH] sparc/build: Make all compiler flags also
clang-compatible
Hi Koakuma,
On Thu, Jun 20, 2024 at 10:56:00PM +0700, Koakuma via B4 Relay wrote:
> From: Koakuma <koachan@...tonmail.com>
>
> Remove flags not supported by clang and make sure that all the flags
> used are portable between clang and GCC.
>
> The reasoning for removing the -fcall-used* ones is as follows:
>
> In the (normal) 32-bit ABI, %g5 and %g7 is normally reserved, and in
> the 64-bit ABI, %g7 is the reserved one.
> Linux turns them into volatile registers by the way of -fcall-used-*,
> but on the other hand, omitting the flags shouldn't be harmful;
> compilers will now simply refuse to touch them, and any assembly
> code that happens to touch them would still work like usual (because
> Linux' conventions already treats them as volatile anyway).
>
> Signed-off-by: Koakuma <koachan@...tonmail.com>
> ---
> Hello~
>
> This changes the CFLAGS for building the SPARC kernel so that it can be
> built with clang, as a follow up from the discussion in this thread:
>
> https://lore.kernel.org/lkml/JAYB7uS-EdLABTR4iWZdtFOVa5MvlKosIrD_cKTzgeozCOGRM7lhxeLigFB1g3exX445I_W5VKB-tAzl2_G1zCVJRQjp67ODfsSqiZWOZ9o=@protonmail.com/T/#u
>
> The changes are removal of various `-fcall-used-*` flags, and changing
> `-mv8plus` to `-mcpu=v9`:
>
> - `-fcall-used-*` flags should be safe to remove; the compiler will
> stop using the registers specified as temporaries, but it is a safe
> change wrt. the ABI. Assembly code can still use those registers
> as needed.
> It does bring a theoretical possible slowdown due to the compiler
> having less registers to work with, but in practice - in my case,
> at least - it seems to not make any difference with daily usage.
>
> - More trivial is to change `-mv8plus` -> `-mcpu=v9`.
> This should be safe too since the kernel seems to require a V9
> processor to run anyway, so I'm changing the flag to one that is
> portable between GCC and clang.
>
> Also, as stated in the thread, building with these changes still result
> in a working kernel, at least for Sun T5120 and qemu virtual machines.
>
> On the LLVM side, the effort for building Linux/SPARC is tracked here:
> https://github.com/llvm/llvm-project/issues/40792
This is really awesome to see, thanks for sending this patch!
I think a good amount of the reasoning below the '---' could probably
make it into the commit message as well but I don't have much of a
vision there, maybe one of the SPARC folks will.
I saw through the LLVM issue above that one other patch is necessary to
fix an issue in the vDSO [1], which I applied in testing this one. I
noticed in applying that change that you appear to be working on 6.1,
which is fine for now, but you'll need another diff once you get to a
newer version, as we stopped using CROSS_COMPILE to set clang's
'--target=' value:
diff --git a/scripts/Makefile.clang b/scripts/Makefile.clang
index 6c23c6af797f..2435efae67f5 100644
--- a/scripts/Makefile.clang
+++ b/scripts/Makefile.clang
@@ -10,6 +10,7 @@ CLANG_TARGET_FLAGS_mips := mipsel-linux-gnu
CLANG_TARGET_FLAGS_powerpc := powerpc64le-linux-gnu
CLANG_TARGET_FLAGS_riscv := riscv64-linux-gnu
CLANG_TARGET_FLAGS_s390 := s390x-linux-gnu
+CLANG_TARGET_FLAGS_sparc := sparc64-linux-gnu
CLANG_TARGET_FLAGS_x86 := x86_64-linux-gnu
CLANG_TARGET_FLAGS_um := $(CLANG_TARGET_FLAGS_$(SUBARCH))
CLANG_TARGET_FLAGS := $(CLANG_TARGET_FLAGS_$(SRCARCH))
With those, I can successfully build a kernel with clang that boots in
QEMU :)
$ make -skj"$(nproc)" \
ARCH=sparc64 \
CC=clang \
CROSS_COMPILE=sparc64-linux-gnu- \
LLVM_IAS=0 \
mrproper defconfig all
$ qemu-system-sparc64 \
-serial mon:stdio \
-display none \
-no-reboot \
-M sun4u \
-cpu 'TI UltraSparc IIi' \
-m 512 \
-append console=ttyS0 \
-initrd sparc64-rootfs.cpio \
-kernel arch/sparc/boot/image
...
[ 1.788544] Run /init as init process
...
Linux version 6.10.0-rc4+ (nathan@...lio-3990X) (ClangBuiltLinux clang version 19.0.0git (https://github.com/llvm/llvm-project a083e50f53f0f9eb9ad0c5b65f3c627cf97043e6), GNU ld (GNU Binutils) 2.42) #1 SMP Fri Jun 21 11:36:18 MST 2024
...
Consider this:
Tested-by: Nathan Chancellor <nathan@...nel.org>
[1]: https://github.com/koachan/linux-clang/commit/c0114bfc7a4f64bc4d3e63eca6582ec827a8e2a2
> ---
> arch/sparc/Makefile | 4 ++--
> arch/sparc/vdso/Makefile | 4 ++--
> 2 files changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/arch/sparc/Makefile b/arch/sparc/Makefile
> index 757451c3ea1d..7318a8b452c3 100644
> --- a/arch/sparc/Makefile
> +++ b/arch/sparc/Makefile
> @@ -29,7 +29,7 @@ UTS_MACHINE := sparc
> # versions of gcc. Some gcc versions won't pass -Av8 to binutils when you
> # give -mcpu=v8. This silently worked with older bintutils versions but
> # does not any more.
> -KBUILD_CFLAGS += -m32 -mcpu=v8 -pipe -mno-fpu -fcall-used-g5 -fcall-used-g7
> +KBUILD_CFLAGS += -m32 -mcpu=v8 -pipe -mno-fpu
> KBUILD_CFLAGS += -Wa,-Av8
>
> KBUILD_AFLAGS += -m32 -Wa,-Av8
> @@ -45,7 +45,7 @@ export BITS := 64
> UTS_MACHINE := sparc64
>
> KBUILD_CFLAGS += -m64 -pipe -mno-fpu -mcpu=ultrasparc -mcmodel=medlow
> -KBUILD_CFLAGS += -ffixed-g4 -ffixed-g5 -fcall-used-g7 -Wno-sign-compare
> +KBUILD_CFLAGS += -ffixed-g4 -ffixed-g5 -Wno-sign-compare
> KBUILD_CFLAGS += -Wa,--undeclared-regs
> KBUILD_CFLAGS += $(call cc-option,-mtune=ultrasparc3)
> KBUILD_AFLAGS += -m64 -mcpu=ultrasparc -Wa,--undeclared-regs
> diff --git a/arch/sparc/vdso/Makefile b/arch/sparc/vdso/Makefile
> index 243dbfc4609d..929140facabf 100644
> --- a/arch/sparc/vdso/Makefile
> +++ b/arch/sparc/vdso/Makefile
> @@ -46,7 +46,7 @@ CFL := $(PROFILING) -mcmodel=medlow -fPIC -O2 -fasynchronous-unwind-tables -m64
> -fno-omit-frame-pointer -foptimize-sibling-calls \
> -DDISABLE_BRANCH_PROFILING -DBUILD_VDSO
>
> -SPARC_REG_CFLAGS = -ffixed-g4 -ffixed-g5 -fcall-used-g5 -fcall-used-g7
> +SPARC_REG_CFLAGS = -ffixed-g4 -ffixed-g5
>
> $(vobjs): KBUILD_CFLAGS := $(filter-out $(RANDSTRUCT_CFLAGS) $(GCC_PLUGINS_CFLAGS) $(SPARC_REG_CFLAGS),$(KBUILD_CFLAGS)) $(CFL)
>
> @@ -86,7 +86,7 @@ KBUILD_CFLAGS_32 += -fno-stack-protector
> KBUILD_CFLAGS_32 += $(call cc-option, -foptimize-sibling-calls)
> KBUILD_CFLAGS_32 += -fno-omit-frame-pointer
> KBUILD_CFLAGS_32 += -DDISABLE_BRANCH_PROFILING
> -KBUILD_CFLAGS_32 += -mv8plus
> +KBUILD_CFLAGS_32 += -mcpu=v9
> $(obj)/vdso32.so.dbg: KBUILD_CFLAGS = $(KBUILD_CFLAGS_32)
>
> $(obj)/vdso32.so.dbg: FORCE \
>
> ---
> base-commit: 92e5605a199efbaee59fb19e15d6cc2103a04ec2
> change-id: 20240620-sparc-cflags-e7f2dbbd4b9d
>
> Best regards,
> --
> Koakuma <koachan@...tonmail.com>
>
>
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